Processors, memory devices, imagers and other types of microelectronic devices are often manufactured on semiconductor workpieces or other types of workpieces. In a typical application, several individual dies (e.g., devices) are fabricated on a single workpiece using sophisticated and expensive equipment and processes. Individual dies generally include an integrated circuit and a plurality of bond-pads coupled to the integrated circuit. The bond-pads provide external electrical contacts on the die through which supply voltage, signals, etc., are transmitted to and from the integrated circuit. The bond-pads are usually very small, and they are arranged in an array having a fine pitch between bond-pads. The dies can also be quite delicate. As a result, after fabricating the dies, they are packaged to protect the dies and to connect the bond-pads to another array of larger terminals that is easier to connect to a printed circuit board. The dies can be packaged after cutting the workpiece to separate the dies (die-level packaging), or the dies can be packaged before cutting the workpiece (wafer-level packaging).
Conventional die-level packaged microelectronic devices include a microelectronic die, an interposer substrate or lead frame attached to the die, and a molded casing around the die. The bond-pads of the die are typically coupled to terminals on the interposer substrate or the lead frame. In addition to the terminals, the interposer substrate also includes ball-pads coupled to the terminals by conductive traces supported in a dielectric material. Solder balls can be attached to the ball-pads in one-to-one correspondence to form a “ball-grid array.” Packaged microelectronic devices with ball-grid arrays are generally higher grade packages having lower profiles and higher pin counts than conventional packages using lead frames.
One process for die-level packaging includes (a) forming a plurality of dies on a semiconductor wafer, (b) cutting the wafer to separate or singulate the dies, (c) attaching individual dies to an interposer substrate, (d) wire-bonding the bond-pads of the dies to the terminals of the interposer substrate, and (e) encapsulating the dies with a suitable molding compound. Mounting individual dies to interposer substrates or lead frames in the foregoing manner can be a time-consuming and expensive process. In addition, forming robust wire-bonds that can withstand the forces involved in molding processes becomes more difficult as the demand for higher pin counts and smaller packages increases. The process of attaching individual dies to interposer substrates or lead frames may also damage the bare dies. These difficulties have made the packaging process a significant factor in the production of microelectronic devices.
Wafer-level packaging is another process for packaging microelectronic devices in which a redistribution layer is formed over the dies before singulating the dies from the workpiece. The redistribution layer can include a dielectric layer and a plurality of exposed pads formed in arrays on the dielectric layer. Each pad array is typically arranged over a corresponding die, and the pads in each array are coupled to corresponding bond-pads of the die by conductive traces extending through the dielectric layer. After constructing the redistribution layer on the wafer, discrete masses of solder paste can be deposited onto the individual pads. The solder paste is then reflowed to form small solder balls or “solder bumps” on the pads. After forming the solder balls, the wafer is singulated to separate the microelectronic devices, and the individual devices can be attached to printed circuit boards or other substrates.
Wafer-level packaging is a promising development for increasing efficiency and reducing the cost of microelectronic devices because individual dies are effectively “pre-packaged” with a redistribution layer before cutting the wafers to singulate the dies. This enables the use of sophisticated semiconductor processing techniques to form smaller arrays of solder balls. Additionally, wafer-level packaging is an efficient process that simultaneously packages a plurality of dies, thereby reducing costs and increasing throughput.
One drawback of conventional wafer-level packaged devices is that the solder joints between the microelectronic device and substrate are generally quite small and have a relatively short life. Specifically, the microelectronic device and the substrate have different coefficients of thermal expansion, and the microelectronic device generates heat during operation. As a result, the microelectronic device and the substrate expand at different rates during operation, which creates stress on the solder joints. This thermal cycling can cause the small solder joints to fail after relatively few cycles.
One existing approach to increase the life of the solder joints includes constructing a stack of two solder balls on each pad of the redistribution layer. The stack of two solder balls increases the length of the solder ball connection and, consequently, the life of the solder joint. Specifically, this method includes forming a plurality of first solder balls on corresponding pads of the redistribution layer, printing a mold compound onto the redistribution layer, and curing the mold compound. During curing, however, the mold compound wicks up and covers the first solder balls. As a result, the method further includes grinding the cured mold material and a portion of the first solder balls to expose a section of the first solder balls, and then placing a plurality of second solder balls on corresponding first solder balls. The wafer may include open spaces with alignment marks to assist in aligning the second solder balls with the first solder balls.
One problem with this approach, however, is that the grinding process scratches the first solder balls and may adversely affect the structural integrity of the balls and the connection between the first and second solder balls. Another problem with this approach is that the tooling must be modified to form the open spaces and alignment marks on the wafer each time a different device is constructed. If the open spaces and alignment marks are not formed on the wafer, the second solder balls may not be accurately aligned with the first solder balls. Accordingly, there is a need to improve the wafer-level packaging process.